.TH std::setprecision 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::setprecision \- std::setprecision

.SH Synopsis
   Defined in header <iomanip>
   /*unspecified*/ setprecision( int n );

   When used in an expression out << setprecision(n) or in >> setprecision(n), sets the
   precision parameter of the stream out or in to exactly n.

.SH Parameters

   n - new value for precision

.SH Return value

   An object of unspecified type such that

     * if out is an object of type std::basic_ostream<CharT, Traits>, the expression
       out << setprecision(n)
          * has type std::basic_ostream<CharT, Traits>&
          * has value out
          * behaves as if it called f(out, n)
     * if in is an object of type std::basic_istream<CharT, Traits>, the expression in
       >> setprecision(n)
          * has type std::basic_istream<CharT, Traits>&
          * has value in
          * behaves as if it called f(in, n)

   where the function f is defined as:

 void f(std::ios_base& str, int n)
 {
     // set precision
     str.precision(n);
 }

.SH Example


// Run this code

 #include <iomanip>
 #include <iostream>
 #include <limits>
 #include <numbers>

 int main()
 {
     constexpr long double pi{std::numbers::pi_v<long double>};

     const auto default_precision{std::cout.precision()};
     constexpr auto max_precision{std::numeric_limits<long double>::digits10 + 1};

     std::cout << "default precision: " << default_precision << '\\n'
               << "maximum precision: " << max_precision << "\\n\\n"
                  "precision: pi:\\n";

     for (int p{0}; p <= max_precision; ++p)
         std::cout << std::setw(2) << p << "  " << std::setprecision(p) << pi << '\\n';

     std::cout << std::setprecision(default_precision); // restore defaults
 }

.SH Output:

 default precision: 6
 maximum precision: 19

 precision: pi:
  0  3
  1  3
  2  3.1
  3  3.14
  4  3.142
  5  3.1416
  6  3.14159
  7  3.141593
  8  3.1415927
  9  3.14159265
 10  3.141592654
 11  3.1415926536
 12  3.14159265359
 13  3.14159265359
 14  3.1415926535898
 15  3.14159265358979
 16  3.141592653589793
 17  3.1415926535897932
 18  3.14159265358979324
 19  3.141592653589793239

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

     DR    Applied to            Behavior as published             Correct behavior
   LWG 183 C++98      setprecision could only be used with streams usable with any
                      of type std::ostream or std::istream         character stream

.SH See also

   fixed
   scientific
   hexfloat     changes formatting used for floating-point I/O
   defaultfloat \fI(function)\fP
   \fI(C++11)\fP
   \fI(C++11)\fP
   precision    manages decimal precision of floating point operations
                \fI(public member function of std::ios_base)\fP
